tKENTON LIMESTONE. 231 



43. 1. ISOTELUS GIGAS.» 



Pl. LX. Figs. 1 a- i; Pi., I. XI, l'ip;s. ;! a - m, &, 4 a b c ; Pl. LXII. Kigs. 1 a i c, & 2 ; and Pl. LXIII. 



Jlsaphtts platycepha/us. Stokes, 'I'rans. Geol. Soc. London, New series, 1822, Vol. i, p. 208, pi. 27. 



Jsoteltis gigns, and I. planus. Dk Kay, 1824, Ann. Lye. Nat. Hist. New-YorU, Vol. i, pag. 170, pi. 13, fii;. 1,2; 



and pi. 13, fig. 1, 2. 

 Isoteliis gigas. Green, Monograpli, 1S32, p. 07 & 03. 

 Isotelus Cyclops, I. megalops, and /. stegojis. Id. lb. p. 09, 70 & 71. 

 Asaphns platycephahts. Bucki.axd, liridgw. Treatise, Vol. ii, pag. 7(i, pi. 45, fig. 12. 

 Isotelus gigus. Vanuxem, Geol. Rep. New-York, 1S42, pag. 40, fig. 1. 



— — Kmjions, Geol. Rep. New-York, 1842, pag. 389, fig. 1. 



Isotelus gigas, and /. planus. Poktlock, Geol. Rep. Londonderry, 1S43, pag. 296, pi. 7, figs. 1, 2 & 3. 

 Compare .isaphus cornigerus, Hrongxiart, Crust, fossiles, 1S22, pag. IS, pi. 4, fig. 10 (not pi. 2, fig. 1 A, b). 

 Brougniartia isotela, Eaton, Geol. Text-Book, 1S32, pi. 2, fig. 19. 

 ,1saplius potriiii, Murchison, Sil. System, 1S30, pag. Gdl, pi. 23, fig. 9. 



Isotelus powisii, Portlock, Op. cit. pag. 297, pi. 0, fig. 1. - ' 



Isvleltis ovaius, I intennedius, I. schrops, and other figures. Portlock, Op. cit. pag. 297 - 299, plates 



0, S, 9 & 10. 

 Isotelus megistos, Locke, .Am. Jour. Science, 1S42, Vol. xlii, p. 300. 

 — — TraiLS. Assoc. Am. Geologists and Naturalists, 1^43, Vol. i, pag. 221, pi. 6. 



General figure oval-oblong, with the sides rather straight ; buckler in the form of " a 

 spherical triangle," obtuse or more or less rounded at the posterior extremities ; cephalic 

 shield convex in the middle, rapidly descending in front and at the sides, margined by a 

 narrow elevated rim or border ; eyes sublunate, prominent, " subpedunculatcd," strongly 

 supported on the inner and concave side by a projection of the glabella; facial suture 

 continuing from the centre of the front, nearly parallel to the margin, until in a line with 

 the eye, when it turns backward, and leaving the eye upon the maxillary portion, turns 

 outwards and backwards, coming out at the base of the shield distant from the angle ; 

 thorax with eight articulations, the middle lobe about once and a half the breadth of the 

 lateral lobes, the longitudinal grooves continued slightly into the Imcklcr, and more dis- 

 tinctly into the caudal shield ; segments of the middle lobe Hat above, those of the lateral 

 lobes with a groove on ibeir upper surface, extending nearly half way to their extremities ; 

 caudal shield of nearly the same form as the buckler, presenting e.xtcrnally some evidence 

 of a trilobate character, sharpened at its upper lateral angles, and having a single trans- 

 verse groove on each of the lateral lobes extending from the axis more than half way to 

 the margin ; entire surface finely punctulated. 



When the crust of the buckler is removed, a narrow shallow groove is visible at the base. 

 In young specimens the caudal extremity is more pointed, and ])resents the marks of eight 

 anchylosed articulations ; in older specimens, these increase in ntimlier, but the external 



* I have adopted the name of De Kay, though aware that the name of Stokes has precedence in point of time. If 

 it be proved that Isotelus cannot be separated from Asaphus by any important characters, we must abandon the 

 genus ; but at present it is well known to all .American naturalists, and a change of name would not bo attended with 

 any beneficial results. 



